layout.css

/* universal selector - zero out all default margin and padding of browser */
* {
	margin:0;
  	padding:0;
	/*The "box-sizing:border-box;" declaration allows us to include the padding and border in an element's total width and height.*/
	box-sizing: border-box;
  }

html {background-color: white;}

body {
  text-align:center ; 
  width:100% ; 
  background-color:gray ;
	font-family: Arial, sans-serif;
  margin:auto ;}

/* Center all headings and paragraphs*/
h1, h2, h3, h4, h5, p {text-align:center;}

header {
	width: 100%;
	background-color:red;
	padding: 5px;
	border: black solid 1px;
	/* positioned based on the user's scroll position */
	position: -webkit-sticky; /* Safari */
  position: sticky;
  top: 0;
	/* When elements are positioned, they can overlap other elements.

The z-index property specifies the stack order of an element (which element should be placed in front of, or behind, the others).

An element can have a positive or negative stack order */
	z-index: 1;
}
/* Clearfix hack */
.clearfix {
overflow:auto;
}

nav {
	background-color:blue;
  float:left;
  width: calc(20% - 10px);
	min-width: 100px;
  height:500px;
  margin:5px;
	padding: 5px;
	border: black solid 1px;
	/* positioned relative to the viewport, which means it always stays in the same place even if the page is scrolled */
	position: fixed;
}

section {
	background-color:yellow ;
  float:left ;
  width:calc(60% - 10px);
	min-width: 100px;
  height: 500px ;
  margin: 5px ;
	padding: 5px;
	border: black solid 1px;
	/* positioned relative to its normal position */
	position: relative;
	left: 20%;	
}

article {
	background-color:green ;
  width:90% ;
  height:400px ;
  margin:auto ;
	/* OVERFLOW property controls what happens to content that is too big to fit into an area.
*/
  overflow: auto;
	padding: 5px;
	border: black solid 1px;
	/* positioned relative to the nearest positioned ancestor */
	position: absolute;
	right: 0px;
}

aside {
	background-color:purple ;
  float:right ;
  width: calc(20% - 10px);
	min-width: 100px;
  height:500px ;
  margin:5px ;
	padding: 5px;
	border: black solid 1px;
}

footer {
	background-color:orange;
	/* use clear to stop the flow of content around a floating element*/
    clear:both ;
	padding: 5px;
	border: black solid 1px;
	/* Static is the default position for HTML elements */
	position: static;
	
}